I've been battling a vibration issue for a while now, and think maybe I've got it tracked down. I have an '84 blazer, 2 1/2" ORD shackle flip, 14 B FF, running 33" tires. The vib used to happen during acceleration, deceleration, and just about all the time. Recently, I swapped out the U-bolts and spring plates for 1 ton (I was running 1 ton u-bolts on 1/2 ton plates) and I lowered the T-case using the 1" spacers. The vibrations have almost gone away now. It is smooth during acceleration and deceleration, but I get a pretty bad whirling around 55mph and above. When I say 'whirling', I mean it is a vibration that comes and goes in time; Almost like a bearing would feel. If I'm doing 60mph, and I shift into neutral, the vib is still present but it becomes constant instead of cycling. When I shift back into drive, it starts the cycling again. Looking at my drive shaft angles, the drive shaft is almost straight in line with the pinion and has a pretty good angle up at the T-case. Would this cause what I am describing? and if so, do I need to shim the rear to bring the pinion down some? Thanks for any advice, Thomas.